Embodiments of our invention relate to hetero phase polymer compositions having three main polymeric components comprising a polypropylene component, a modifier component and a compatibilizer component. The compatibilizer component imparts a greater degree of compatibility between the polypropylene component and modifier component phases, yielding improved physical properties, notably impact strength. Non-polymeric components may also be present in the compositions.
The term hetero phase polymer compositions refers to the presence of at least two phases: a continuous or matrix phase and a discontinuous or disperse phase distributed within the matrix phase.
Hetero phase polymer compositions include compositions generally referred to as Thermoplastic Olefin (TPO) Compositions, which are blends of polypropylene, modifier and optionally non-polymeric components such as fillers and other compounding ingredients. TPOs are multiphase polymer blends where the polypropylene forms a continuous matrix phase and the modifier component, generally derived from an ethylene containing interpolymer, is the dispersed component. The polypropylene matrix imparts tensile strength and chemical resistance to the TPO, while the ethylene polymer imparts flexibility and impact resistance. Generally speaking, TPOs have a dispersed phase which is not, or only modestly, cross-linked.
Hetero phase polymer compositions also include compositions generally referred to as thermoplastic vulcanizates (TPVs), which are also blends of polypropylene, modifier and optionally non-polymeric components such as fillers and other compounding ingredients, with the exception that thermoplastic vulcanizates are multiphase polymer blends where the dispersed modifier component is cross-linked or “vulcanized” to provide a rubber-like resilience to the composition.
Traditionally, highly amorphous, very low density ethylene-propylene copolymers (EP) and ethylene-propylene-diene terpolymers (EPDM) have been used as the modifier component in hetero phase compositions, these EP or EPDMs generally have a high molecular weight expressed in Mooney units. Recently, other ethylene-alpha olefin copolymers have been used, especially very low density ethylene-butene, ethylene-hexene and ethylene-octene copolymers which generally have a lower molecular weight expressed in Melt Index units. The density of these latter polymers is generally less than 0.900 g/cm3, indicative of some residual crystallinity in the polymer. The major market for TPOs is in the manufacture of automotive parts, especially bumper fascia. Other applications include automotive interior components such as door skin, air bag cover, side pillars and the like. These parts are generally made using an injection molding processes. To increase efficiency and reduce costs it is necessary to decrease molding times and reduce wall thickness in the molds. To accomplish these goals, manufacturers have turned to high melt flow polypropylenes (Melt Flow Rate>35 g/10 min.). These high melt flow rate (MFR) resins are low in molecular weight and consequently difficult to toughen, resulting in products that have low impact strength.
U.S. Pat. No. 6,245,856 suggests hetero phase compositions of the TPO type containing a polypropylene component, modifier component and compatibilizer component. The polypropylene component is generally described as a polypropylene homopolymer having a relatively high MFR. This document refers to the possible use of polypropylene impact copolymers without restricting the nature and amount of the other components. The modifier component is described as an elastomer component which can be an ethylene alpha olefin copolymer or an ethylene alpha olefin diene terpolymer. The density of the modifier component is stated to range from 0.85 to 0.90 g/cm3.
There is a need therefore for TPO manufacturers to be able to broaden the scope of polymers available to manufacture end use items with a better balance between the performance of the hetero phase composition in its end use, the processability during conversion of the molten compositions into the end use article and the cost of providing those properties.
Other background references include U.S. Pat. No. 5,959,030 to Berta, U.S. Pat. No. 6,245,856 to Kaufman et al., U.S. Pat. No. 6,232,402 to Demeuse, WO 97/20888, and EP 0 792 914.